Functions & Working of ICMP (Internet Control Message Protocol)

Welcome to the Network Engineer training section by CoreNetworkZ Tech Solutions. As a trusted provider of technical training for IT professionals, we are excited to share a key topic essential for both CCNA and JNCIA certification paths.

Today, I will explain ICMP and related questions asked in the CCNA and JNCIA examinations.

John Samuel, a CCNA student at CoreNetworkZ Tech Solutions, asked this question in one of our induction classes.

ICMP (Internet Control Message Protocol) is a network layer protocol that generates error reports when facing problems with receiving or delivering packets.

It is a connectionless protocol with the primary duty of error reporting.

Ashiq Muhammed, a B.Tech 3^rd year student from Delhi, asked, "What is the importance of ICMP in a network?"

Let me answer Ashiq in this section. The primary reason for the importance of ICMP is its error reporting and the support for ping and tracert.

ICMP helps troubleshoot the connection between two hops.

Sonya Merchant, a client of CoreNetworkZ Tech Solutions, asked, "Why do you network engineers need this protocol?"

Let me explain one critical duty of a network engineer: network troubleshooting. Checking the connectivity between two devices is the primary focus of network troubleshooting.

We have many utilities to check the connectivity and availability of the destination device.

But all these utilities are based on ICMP, as defined in RFC 792.

The Internet Control Message Protocol (ICMP) is used to transmit error and control messages between devices that support the TCP/IP protocol suite.

ICMP operates behind utilities such as ping and tracert. Now, I will answer a question asked by a reader in the comment section.

Akhil Raj, a regular reader of CoreNetworkZ Tech Solutions, asked, "Can you explain the functions of ICMP and provide a list of the error messages it generates?"

Let us now explore the key functions of the Internet Control Message Protocol.

Let me answer the first part of Akhil's question here.

1. Send echo requests from a TCP/IP device to another.

2. Responds to Echo request.

3. It helps to find routing path issues.

4. ICMP echo message verifies the availability of the destination.

In this section, I will answer the second part of Akhil Raj's question. He asked about the list of messages generated by ICMP.

1. Address Request

2. Address reply

3. Destination unreachable

   To learn more about the Destination Unreachable message generated by the Internet Control Message Protocol, visit the links below.

   • Destination Net Unreachable
   • Destination Host Unreachable

4. Subnet Mask Request

5. Time Exceeded

You have already learned about the Internet Control Message Protocol (ICMP). Now, let us explore its practical applications in real-world scenarios.

I have a home network with several devices connected to the Internet via an ISP-provided modem.

Additionally, I manage a small office network with ten computers. Let us consider a hypothetical connectivity issue between my home computer and the office network.

So, how should I begin troubleshooting if I am unable to access the office network from home?

The first step is to ping one of the devices on the office network from my personal computer at home.

It sends ICMP echo request packets from my home computer to the office computer. Here, we can observe two key functions of ICMP at work:

1. It tests the connectivity between my home computer and office computer.

2. It also generates error messages (if a connectivity issue occurs).

I believe my students enjoyed today's tutorial. To pass the CCNA and JNCIA examinations, I recommend you read the following additional resources.

1. https://community.cisco.com/t5/networking-knowledge-base/icmp-internet-control-message-protocol/ta-p/3116636
2. https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/7/html/security_guide/sec-managing_icmp_requests
3. https://www.juniper.net/documentation/us/en/software/junos/transport-ip/topics/topic-map/icmp.html
4. https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-adts/b78f5642-0cb3-4d37-9a71-e08731a5d000